The Goldilocks effect: female geladas in mid-sized groups have higher fitness

Abstract

The cost–benefit ratio of group living is thought to vary with group size: individuals in ‘optimally sized’ groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here, we present evidence for optimal group size that maximizes female fitness in a population of geladas ( Theropithecus gelada ). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized groups exhibited the highest reproductive performance. This group size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized groups are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species.

Reproductive plasticity in response to the changing cluster size during the breeding period: a case study in a spider mite

“…Animals are predicted to live in clusters when the benefits of cluster living outweigh the costs (Alexander 1974 ; Bilde et al 2007 ; Guindre-Parker et al 2020 ; Tinsley Johnson et al 2021 ). In cluster-living species, cluster size is an important component of social environment that influences individuals’ behaviour (Prokopy and Duan 1998 ; Majolo et al 2008 ; Krams et al 2009 ; Le Goff et al 2010 ; Fryxell and Berdahl 2018 ) and physiology (Lihoreau and Rivault 2008 ; Clotuche et al 2014 ; Markham et al 2015 ; Markham and Gesquiere 2017 ; Rudolph et al 2019 ), altering their life-history traits (Prokopy and Reynolds 1998 ; Bilde et al 2007 ; Borries et al 2008 ; Le Goff et al 2010 ; Chapman and Valenta 2015 ; Vanthournout et al 2016 ; Tinsley Johnson et al 2021 ). Some fitness components are expected to increase with increasing cluster size, for example, the reduction of predation risk (Spieler 2003 ; Morrell and James 2008 ; Yano 2012 ; Saito and Zhang 2017 ) and increase in reproductive success (Snead and Alcock 1985 ; Prokopy and Reynolds 1998 ; Le Goff et al 2010 ; Pérez-González et al 2010 ; Bonsignore and Jones 2014 ).…”

“…Some fitness components are expected to increase with increasing cluster size, for example, the reduction of predation risk (Spieler 2003 ; Morrell and James 2008 ; Yano 2012 ; Saito and Zhang 2017 ) and increase in reproductive success (Snead and Alcock 1985 ; Prokopy and Reynolds 1998 ; Le Goff et al 2010 ; Pérez-González et al 2010 ; Bonsignore and Jones 2014 ). However, living in a large cluster may also reduce fitness because of the interference and food competition among group members (Bilde et al 2007 ; Estevez et al 2007 ; Grove 2012 ; Wong et al 2013 ; Li and Zhang 2021 ; Tinsley Johnson et al 2021 ).…”

“…However, cluster size often changes while individuals are reproducing due to frequent deaths, births, aggregation (immigration) and dispersal (emigration) (Roeder 1992 ; Roff 1992 ; Stearns 1992 ; Price and Hunter 1995 ; Bowman et al 2002 ; Schausberger et al 2021 ). As a result, animals must have developed reproductive plasticity to adapt to dynamic social environment to maximize the fitness of their offspring and their own (Ross et al 2013 ; Radwan et al 2014 ; De Roissart 2016 ; Weerawansha et al 2020 , 2022a , b ; Tinsley Johnson et al 2021 ). To date, it is still unknown how changes of cluster size during female reproductive phase affect their reproductive strategies.…”